A–6

Glossary

Appendix A

PID Loop

Proportional - Integral-Derivative - A mathematical model used for

 

process control. A process controller maintains a process variable

 

(PV) at a setpoint (SP) by using its PID algorithm to compensate for

 

dynamic conditions and vary its output to drive the PV toward the

 

desired value. For variable-frequency drives, the process variable is

 

the motor speed. See also Error.

Process Variable

A physical property of a process that is of interest because it affects

 

the quality of the primary task accomplished by the process. For an

 

industrial oven, temperature is the process variable. See also PID

 

Loop and Error.

PWM

Pulse-width modulation: A type of AC adjustable frequency drive

 

that accomplishes frequency and voltage control at the output

 

section (inverter) of the drive. The drive output voltage waveform is

 

at a constant amplitude, and by “chopping” the waveform (pulse-

 

width-modulating), the average voltage is controlled. The chopping

 

frequency is sometimes called the Carrier Frequency.

Reactance

The impedance of inductors and capacitors has two components.

 

The resistive part is constant, while the reactive part changes with

 

applied frequency. These devices have a complex impedance

 

(complex number), where the resistance is the real part and the

 

reactance is the imaginary part.

Rectifier

An electronic device made of one or more diodes that converts AC

 

power into DC power. Rectifiers are usually used in combination

 

with capacitors to filter (smooth) the rectified waveform to closely

 

approximate a pure DC voltage source.

Regenerative Braking

A particular method of generating reverse torque to a motor, an

 

inverter will switch internally to allow the motor to become a gener-

 

ator and will either store the energy internally, deliver the braking

 

energy back to the main power input, or dissipate it with a resistor.

Regulation

The quality of control applied to maintain a parameter of interest at

 

a desired value. Usually expressed as a percent (±) from the

 

nominal, motor regulation usually refers to its shaft speed.

Reverse Torque

The torque applied in the direction opposite to motor shaft rotation.

 

As such, reverse torque is a decelerating force on the motor and its

 

external load.

Rotor

The windings of a motor that rotate, being physically coupled to the

 

motor shaft. See also Stator.

Saturation Voltage

For a transistor semiconductor device, it is in saturation when an

 

increase in input current no longer results in an increase in the

 

output current. The saturation voltage is the voltage drop across the

 

device. The ideal saturation voltage is zero.